Various documentation improvements

* Improve documentation page
  * Move instructions on how to improve the docs to the bottom of the page.
  * Promote using and customizing WET to be the top options.
  * Use more direct language and links in first paragraph.
  * Remove repetition of "Getting started".
* Getting started improvements
  * Move using and customizing WET to top
  * Remove "Step 0" of creating a GitHub account, which is not needed in two out of four activities
  * Improve clarity of instructions
* Highlight the current stable version on the downloads page
* Correct typo on pull request page
* Correct typos on bugs page